Field of the Invention
The invention relates generally to the field of marine seismic surveying. More particularly, the invention relates to methods for correcting marine seismic sensor array response for variations in array geometry during operation.
Background Art
Marine seismic surveying systems are used to acquire seismic data from Earth formations below the bottom of a body of water, such as a lake or the ocean. Marine seismic surveying systems typically include a seismic vessel having onboard navigation, seismic energy source control, and data recording equipment. The seismic vessel is typically configured to tow one or more streamers through the water. At selected times, the seismic energy source control equipment causes one or more seismic energy sources (which may be towed in the water by the seismic vessel or by another vessel) to actuate. Signals produced by various sensors on the one or more streamers are ultimately conducted to the recording equipment, where a record with respect to time is made of the signals produced by each sensor (or groups of such sensors). The recorded signals are later interpreted to infer the structure and composition of the Earth formations below the bottom of the body of water.
The one or more streamers are in the most general sense long cables that have seismic sensors disposed at spaced apart positions along the length of the cables. A typical streamer can extend behind the seismic vessel for several kilometers. Because of the great length of the typical streamer, the streamer may not travel entirely in a straight line behind the seismic vessel at every point along its length due to interaction of the streamer with the water and currents in the water, among other factors.
The quality of images of the Earth's subsurface produced from three dimensional seismic surveys is affected by how well the positions of the individual sensors on the streamers are known and controlled. The quality of images generated from the seismic signals also depends to an extent on the relative positions of the seismic receivers being maintained throughout the seismic survey. Various devices are known in the art for positioning streamers laterally and/or at a selected depth below the water surface. U.S. Pat. No. 5,443,027 issued to Owsley et al., for example, describes a lateral force device for displacing a towed underwater acoustic cable that provides displacement in the horizontal and vertical directions. The device has a hollow spool and a rotationally mounted winged fuselage. The hollow spool is mounted on a cable with cable elements passing therethrough. The winged fuselage is made with the top half relatively positively buoyant and the bottom half relatively negatively buoyant. The winged fuselage is mounted about the hollow spool with clearance to allow rotation of the winged fuselage. The difference in buoyancy between the upper and lower fuselage maintains the device in the correct operating position. Wings on the fuselage are angled to provide lift in the desired direction as the winged fuselage is towed through the water. The device disclosed in the Owsley et al. patent provides no active control of direction or depth of the streamer, however.
U.S. Pat. No. 6,011,752 issued to Ambs et al. describes a seismic streamer position control module having a body with a first end and a second end and a bore therethrough from the first end to the second end for receiving a seismic streamer. The module has at least one control surface, and at least one recess in which is initially disposed the at least one control surface. The at least one control surface is movably connected to the body for movement from and into the at least one recess and for movement, when extended from the body, for attitude adjustment. Generally, the device described in the Ambs et al. patent is somewhat larger diameter, even when closed, than the streamer to which it is affixed, and such diameter may become an issue when deploying and retrieving streamers from the water.
U.S. Pat. No. 6,144,342 issued to Bertheas et al. describes a method for controlling the navigation of a towed seismic streamer using “birds” affixable to the exterior of the streamer. The birds are equipped with variable-incidence wings and are rotatably fixed onto the streamer. Through a differential action, the wings allow the birds to be turned about the longitudinal axis of the streamer so that a hydrodynamic force oriented in any given direction about the longitudinal axis of the streamer is obtained. Power and control signals are transmitted between the streamer and the bird by rotary transformers. The bird is fixed to the streamer by a bore closed by a cover. The bird can be detached automatically as the streamer is raised so that the streamer can be wound freely onto a drum. The disclosed method purportedly allows the full control of the deformation, immersion and heading of the streamer.
Systems for determining relative positions of seismic streamers in the water are disclosed, for example, in U.S. Pat. No. 7,376,045 issued to Falkenberg et al. (“the '045 patent”) and assigned to the assignee of the present invention. A system disclosed in the '045 patent includes a plurality of acoustic transmitters, mounted inside the streamers, adapted to transmit broadband signals having low cross-correlation between the signals of different transmitters; a plurality of acoustic receivers, mounted inside the streamers, adapted to receive the signals from the transmitters; at least one processor adapted to cross-correlate the signals received at the receivers with copies of transmitter signals to determine identities of the transmitters of the received signals and to determine travel times of the received signals; and a main processor adapted to convert the travel times to distances between the identified transmitters and the receivers and to determine relative positions of the streamers from the distances.
There continues to be a need for devices and methods for marine seismic streamers to determine and/or maintain depth and heading of the streamers along their length in order to improve the quality of seismic surveys.